Group III nitride semiconductor substrate, substrate for group III nitride semiconductor device, and methods of making same

ABSTRACT

A III group nitride semiconductor substrate according to the present invention is fabricated by forming a metal film or metal nitride film  2′  with mesh structure in which micro voids are provided on a starting substrate  1 , and growing a III group nitride semiconductor crystal layer  3  via the metal film or metal nitride film  2′.

The present application is a divisional of U.S. application Ser. No.11/431,106, filed May 10, 2006, the entire contents of which isincorporated herein by reference.

This application is based on Japanese patent application No.2003-358350, and a whole content of this Japanese application isreferred and introduced in this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a III group nitride semiconductor substrate, asubstrate for a III group nitride semiconductor device and fabricationmethods thereof.

2. Description of the Related Art

A GaN-based compound semiconductor such as gallium nitride (GaN), indiumgallium nitride (InGaN), and aluminum gallium nitride (AlGaN) comes intothe limelight as a material for a blue light emitting diode (LED) and alaser diode (LD). In addition, with making use of features such as heatresistance, environment characteristics resistance of the GaN-basedcompound semiconductor, the application development of an element for anelectronic device using the GaN-based compound semiconductor begins.

In general terms, as a method for fabricating a GaN-based compoundsemiconductor device, a method for epitaxially growing a GaN-basedcrystal on a sapphire substrate is used. However, a single crystal filmof GaN cannot be directly grown on the sapphire substrate, since thesapphire substrate is different in lattice constant from GaN. So as tosolve this problem, a method of growing a buffer layer of AlN or GaN ata low-temperature of around 500° C. on a sapphire substrate once,relaxing a lattice strain by the low-temperature growth buffer layer,and growing GaN thereon at a high temperature of around 1000° C., isinvented and placed in practical use broadly (For example, JapanesePatent Laid-Open No. 63-188983 (Patent document 1)). The single crystalepitaxial growth of GaN can be achieved by using the low-temperaturegrowth nitride layer as the buffer layer.

However, in this technique, the crystallinity of the GaN crystal grownat a high temperature is sensitive to the thickness and thecrystallinity of the low-temperature growth buffer layer, so that it isdifficult to grow the GaN crystal with a good repeatability. Inaddition, since a growth temperature must be varied in the crystalgrowth, there are disadvantages in that the increase/decrease andstabilization of the temperature take a longer time and so on. Further,the epitaxial growth of the GaN single crystal on the sapphire substratecan be realized by using the low-temperature growth buffer layer,however, it is assumed that innumerable defects occurring in the GaN dueto the lattice-mismatch between the substrate and the crystal become theobstacle in fabricating a GaN-based LD.

In recent years, some techniques for reducing the crystal dislocationdensity due to the difference in lattice constant between the sapphireand the GaN were reported, for example, an ELO (Epitaxial LateralOvergrowth) method (Appl. Phys. Lett. 1997, Vol. 71, No. 18, p. 2638),and FIELO (Facet-Initiated Epitaxial Lateral Overgrowth) method (Jpn. J.Appl. Phys. 1999, Vol. 38, p. L184), as well as a Pendio-epitaxy (MRSInternet J. Nitride Semicond. Res. 1999, 4S1, G3, 38), and GaN epitaxialwafers with significantly improved crystallinity can be obtained.However, as for the ELO method, etc., there is a disadvantage in thatcomplicated process such as photolithography process, etching process isrequired. Further, according to these methods, there is a disadvantagein that the dislocation distribution in the GaN becomes heterogeneousunless a GaN thick film having a thickness more than dozens ofmicrometers is grown.

In recent years, techniques of growing a GaN thick film on a substrateby HVPE (Hydride Vapor Phase Epitaxy) method and cutting out aself-standing substrate of GaN are disclosed (Japanese Patent Laid-OpenNo. 2000-012900, Japanese Patent Laid-Open No. 2000-022212, JapanesePatent Laid-Open No. 2000-252217 (patent documents 2 to 4), etc.). Inthese techniques, a technique for fabricating a GaN substrate once andgrowing a GaN crystal ingot by using this as a seed crystal thereafteris adopted, however, so as to simplify the process and to reduce themanufacturing cost of the GaN substrate, a technique for conducting ahetero epitaxial growth of a GaN thick film on a foreign substrate suchas sapphire substrate and cutting out a self-standing substrate of theGaN directly therefrom is desirable. However, according to aconventional growth method using the low-temperature growth bufferlayer, the relaxation of the strain due to the difference in latticeconstant between the starting substrate and the GaN is not enough,therefore, for example, when growing the GaN on the sapphire substrateto have a thickness of about 100 μm, the GaN is naturally cracked duringthe growth, so that it is difficult to grow the GaN to have an enoughthickness to cut out a self-standing substrate therefrom.

It has been known for a long time that micro voids are formed to have amesh structure when a thin metal film is heated. For example, it isreported that micro voids with mesh structure are formed in a thin filmof nickel, gold, copper, etc. by heating in M. L. Gimpl, A. D. McMaster,and N. Fuschillo, J. Appl. Phys. 1964, Vol. 35, p. 3572 (non-patentdocument 1) and L. Bachmann, D. L. Sawner, and B. M. Siegel, J. Appl.Phys. 1965, Vol. 36, p. 304 (non-patent document 2). Such mesh structureis conspicuously observed particularly in a film formed by the vacuumdeposition method. However, a technique for conducting a crystal growthof a III group nitride semiconductor via a film with mesh structure thusobtained is not known.

Techniques for growing a III group nitride semiconductor crystal viatitanium metal, titanium nitride, zirconium nitride, or hafnium nitrideare disclosed in Japanese Patent Laid-Open No. 11-195814, JapanesePatent Laid-Open No. 2000-049092, and Japanese Patent Laid-Open No.2000-323753 (patent documents 5 to 7), and it is disclosed that the GaNcrystal growth is possible even if the low-temperature growth bufferlayer is not provided, in the Japanese Patent Laid-Open No. 2000-323753(patent document 8). However, it is specified that it is preferable thatthe low-temperature growth buffer layer intervenes and it is difficultto obtain a high quality GaN crystal by the epitaxial growth, when onlythe titanium nitride, etc. intervenes.

Technique for growing III group nitride semiconductor crystal byproviding a mask of high melting thin metal film is disclosed inJapanese Patent Laid-Open No. 2000-114178 (patent document 9). However,this mask is fabricated by making full use of the photolithographymethod, and an aperture width of a window in a mask is limited toseveral micrometers due to restrictions imposed by processing accuracy.In the case of growing the III group nitride semiconductor crystal bymeans of this mask, a number of crystal growth nucleuses are generatedfrom inside the windows in the mask, and effect for reducing the crystaldislocation can be obtained, however, the insertion of thelow-temperature buffer layer becomes indispensable.

[Patent document 1] Japanese Patent Laid-Open No. 63-188983 bulletin

[Patent document 2] Japanese Patent Laid-Open No. 2000-012900 bulletin

[Patent document 3] Japanese Patent Laid-Open No. 2000-022212 bulletin

[Patent document 4] Japanese Patent Laid-Open No. 2000-252217 bulletin

[Patent document 5] Japanese Patent Laid-Open No. 11-195814 bulletin

[Patent document 6] Japanese Patent Laid-Open No. 2000-049092 bulletin

[Patent document 7] Japanese Patent Laid-Open No. 2000-323753 bulletin

[Patent document 8] Japanese Patent Laid-Open No. 2000-114178 bulletin

[Patent document 9] Japanese Patent Laid-Open No. bulletin

[Non-patent document 1] M. L. Gimpl, A. D. McMaster, and N. Fuschillo,“Journal of Applied Physics” (Journal of Applied Physics), 1964, Vol.35, p. 3572

[Non-patent document 2] L. Bachmann, D. L. Sawner, and B. M. Siegel,“Journal of Applied Physics” (Journal of Applied Physics), 1965, Vol.36, p. 304

SUMMARY OF THE INVENTION

Accordingly, an object of the invention is to provide a method forfabricating a III group nitride semiconductor substrate without the needof forming a low-temperature growth buffer layer and changing thetemperature during the growth.

Further, another object of the invention is to provide a III groupnitride semiconductor crystal grown by hetero epitaxial growth on astarting substrate, in which the strain with the starting substrate ismore relaxed and the crystal dislocation is less than conventionalmethods.

Still further, still another object of the invention is to provide a IIIgroup nitride semiconductor substrate which allows the relaxation of thestrain with the starting substrate during the hetero epitaxial growth ofthe III group nitride semiconductor crystal and allows the crystalgrowth of the thick film in which the self-standing substrate can be cutout.

In addition, a further object of the invention is to provide a III groupnitride semiconductor substrate, a substrate for a III group nitridesemiconductor device, and fabrication methods thereof, provided with aIII group nitride semiconductor crystal with high quality in which asurface is a nonpolar surface.

As a result of serious researches in view of the above objects, theInventors of the present invention found that strain with a startingsubstrate can be relaxed by forming a metal film or a metal nitride filmwith mesh structure in which innumerable micro voids are provided on astarting substrate and growing III group nitride semiconductor crystalvia the metal film or metal nitride film, so that a III group nitridesemiconductor crystal with less dislocation can be obtained, therebyinventing the present invention. In other words, the first method forfabricating a III group nitride semiconductor substrate according to thepresent invention comprises the steps of forming a metal film on astarting substrate, conducting a heat treatment for the startingsubstrate on which the metal film is formed to form innumerable microvoids which extend from a surface of the metal film to a surface of thestarting substrate in the metal film, and growing a III group nitridesemiconductor crystal via the metal film on the heat treated startingsubstrate.

The second method for fabricating a III group nitride semiconductorsubstrate according to the present invention comprises the steps offorming a metal film on a starting substrate, conducting a heattreatment for the starting substrate on which the metal film is formedin an atmosphere of gas containing a nitrogen element at a temperatureof 800° C. or more to form a metal nitride film by nitriding the metalfilm and to form innumerable micro voids which extend from a surface ofthe metal film to a surface of the starting substrate in the metalnitride film, and growing a III group nitride semiconductor crystal viathe metal nitride film on the heat treated starting substrate.

It is preferable that the starting substrate comprises a sapphire singlecrystal. As a preferred starting substrate, a (0001) face of a hexagonalsystem single crystal, in concrete, a single crystal substrate ofsapphire (Al₂O₃), zinc oxide (ZnO), etc., or a (111) face of cubicsystem single crystal, in concrete, a single crystal substrate ofsilicon (Si), gallium arsenide (GaAs), etc are proposed. In addition,for obtaining the III group nitride semiconductor crystal in which asurface is a nonpolar surface, a surface of the starting substrate (aforming surface of a metal film or a metal nitride film) is used as anonpolar forming surface. When the starting substrate is sapphire, ar-plane is the nonpolar forming surface. Herein, the nonpolar surface ofthe III group nitride semiconductor crystal is an a-plane or m-planecrystal plane.

The metal film to be formed on the starting substrate preferablycomprises at least one element selected from a group composed ofscandium (Sc), yttrium (Y), titanium (Ti), zirconium (Zr), hafnium (Hf),vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum(Mo), tungsten (W), rhenium (Re), iron (Fe), ruthenium (Ru), osmium(Os), cobalt (Co), rhodium (Rh), iridium (Ir), nickel (Ni), palladium(Pd), manganese (Mn), copper (Cu), platinum (Pt) and gold (Au). Thesemetal films may be a multilayer film composed of a plurality of metalsor an alloy film.

Heat treatment condition for forming the voids in the metal film toprovide the mesh structure can be appropriately changed in accordancewith the kind of the metal, since an optimum value for the heattreatment depends on the kind of the metal. When the heat treatment isconducted simultaneously with the nitriding process, it is preferablyconducted at a temperature of 800° C. or more. The gas containing thenitrogen elements to be used for the nitriding process of the metal filmcomprises preferably at least one element selected from a group composedof nitrogen, ammonia, hydrazine and dimethylhydrazine. These gases mayfurther comprise hydrogen.

It is required that the metal film or the metal nitride film is thinnerthan 200 nm so as to provide voids that penetrate to the surface of thestarting substrate. On the other hand, if the metal film or the metalnitride film is thinner than 2 nm, for example, a part of the metal filmmay be torn in the heat treatment or the like, so that it becomesdifficult to homogeneously form the film with mesh structure on anentire surface of the substrate Therefore, the thickness of the metalfilm or the metal nitride film is preferably 2 to 200 nm, and morepreferably 10 to 100 nm.

It is preferable that the process for growing a III group nitridesemiconductor crystal is conducted at a temperature of 900° C. or more,and more preferably at a temperature of 950° C. or more so as to keepthe quality of the III group nitride semiconductor crystal high. Whenthe crystal growth temperature is lower than 900° C., the dislocation inthe crystal increases, so that it becomes difficult to keep the highquality. While the high quality crystal is colorless and transparent,when the dislocation in the crystal increases, a phenomenon that thecrystal is colored by yellow to brown is observed. As for an upper limitof the crystal growth temperature, 1180° C. is preferable to avoid theheat decomposition of the crystal.

An area of each opening of the voids provided in the metal film or themetal nitride film is preferably 1×10⁻⁹ m², and more preferably 1×10⁻¹⁵to 1×10⁻¹² m². This void plays a role to provide a nuclei generatingsite at an initial growth stage in the crystal growth of the III groupnitride semiconductor, so that it is preferable to generate a crystalnuclei from one void as few as possible, and it is more preferable togenerate a single crystal nuclei from a single void. When the area ofeach opening is greater than 1×10⁻⁹ m², numerous crystal nucleuses aregenerated from the void, so that the strain relaxation effect by virtueof a porous film is decreased, as a result, it is often that a crack mayoccur in the crystal during the crystal growth or that the grown crystalbecomes polycrystal. On the other hand, if the area of each opening issmaller than 1×10⁻¹⁶ m², it does not function as the nuclei generatingsite, so that the strain relaxation effect cannot be obtained. It ispreferable that the voids are substantially homogeneously distributed inthe metal film or the metal nitride film. According to this, the IIIgroup nitride semiconductor crystal can be homogeneously grown on thestarting substrate. When the distribution of the voids is heterogeneous,a thickness distribution of the III group nitride semiconductor crystalor a dislocation distribution of the crystal becomes heterogeneous atthe surface of the substrate, so that the crack may occur in the crystalduring the growth, and it may cause that the grown crystal becomes apolycrystal.

A total area of the opening of the void provided in the metal film orthe metal nitride film is preferably 10 to 90 area %, and morepreferably 30 to 70 area % for a surface area of the starting substrate.When the total area of the opening is smaller than 10 area % for thesurface area of the starting substrate, even if the III group nitridesemiconductor crystal is grown, the crystal surface may not beflattened, or the nuclei may be generated from a region other than theinside of the void, so that the crystal surface maybe roughened or thegrown crystal may become polycrystal. On the other hand, the area of theopening of the void is greater than 90 area %, it becomes difficult tomaintain the configuration of the porous film, thereby producing afactor obstructing the single crystal growth of the III group nitridesemiconductor.

A film thickness of the III group nitride semiconductor crystal ispreferably 500 nm or more, and it is desirable that the surface thereofis substantially flattened. By providing the film thickness of 500 nm ormore, adjacent ones of the initial growth nucleuses in the III groupnitride semiconductor crystal, that are generated from the voidsprovided in the metal film or the metal nitride film, are bound to eachother, so that the entire surface can be flattened. An upper limit ofthe film thickness of the III group nitride semiconductor crystal is notlimited in particular. When the III group nitride semiconductor crystalis thinner than 500 nm, a lot of pits or steps appear on the crystalsurface, and it may become a great obstacle for fabricating a device byusing the obtained crystal.

Additionally, in case where the surface of the III group nitridesemiconductor crystal is the nonpolar surface, it is also a preferredembodiment that such III group nitride semiconductor crystal constitutesa device structure comprising a plurality of III group nitridesemiconductor epitaxial layers, and in that case, the III group nitridesemiconductor crystal has a thickness of an ordinary epitaxial layer forthe device.

The crystal growth of the III group nitride semiconductor is preferablyconducted by using any one of MOVPE (Metal Organic Vapor Phase Epitaxy)method and HVPE (Hydride Vapor Phase Epitaxy) method, or the combinationthereof. Particularly in the initial stage of the crystal growth, it ispreferable to use the MOVPE method in which the crystal growth speed isrelatively slow and that the generation of the initial growth nuclei canbe obtained enough, and when the crystal growth is once begun, it ispreferable to use the HVPE method in which high crystal growth speed canbe obtained. In addition, it is also preferable to use MOHVPE method inwhich the advantages of both methods are combined.

It is preferable to selectively generate the nuclei from the inside ofthe voids formed in the metal film or the metal nitride film at theinitial growth stage in the process for growing the III group nitridesemiconductor crystal. When the micro voids provided in the metal filmor the metal nitride film play a role to provide the nuclei generatingsite at the initial growth stage when conducting the crystal growth ofthe III group nitride semiconductor. Namely, since there is a surfaceenergy difference between the metal film or the metal nitride film andthe starting substrate, materials that extend to a surface of the metalfilm or the metal nitride in the crystal growth of the III group nitridesemiconductor move on the starting substrate within the void because ofthe surface migration, so that crystal growth nuclei is selectivelyformed there. By conducting the crystal growth of the III group nitridesemiconductor via the metal film or the metal nitride film provided withthe micro voids, it is possible to control the density of the initialnuclei generation of the III group nitride semiconductor crystal withoutdepending on the lattice constant of the starting substrate, so that itis possible to conduct the crystal growth in which the occurrence ofdislocation can be significantly suppressed compared with the case wherethe metal film or metal nitride film is not provided. In addition, themetal film or the metal nitride film having the micro voids has aneffect of relaxing the strain due to the lattice mismatch between thestarting substrate and the III group nitride semiconductor crystal anddue to a difference in linear expansion coefficients therebetween, as aresult, it is possible to provide the III group nitride semiconductorcrystal substrate in which the occurrence of the crystal dislocation islow and warping is small. In addition, even if a thick film crystal of 1mm or more is grown, the crack will not occur in the crystal.Accordingly, it is possible to fabricate a III group nitridesemiconductor self-standing substrate by growing a large-sized crystalof 1 mm or more and cutting out a part of the III group nitridesemiconductor crystal which is grown.

According to material of the starting substrate or material of themetal, it is expected that the initial growth nuclei of the III groupnitride semiconductor crystal may be selectively generated on the metalfilm or metal nitride film. For this case, the generation of the nucleiwill not occur from the inside of the voids contrary to theaforementioned case, and the crystal nuclei is generated from a limitedregion on the metal film or the metal nitride film, and the crystalgrowth advances in the manner that the III group nitride semiconductorcrystal is over-grown on the micro voids of the metal film or the metalnitride film, as result, a high quality crystal can be grown withoutusing the low-temperature buffer layer similarly to the case where thenuclei is selectively generated within the void. For this case, thevoids provided in the metal film or the metal nitride film functions torelax the strain due to the lattice mismatch between the startingsubstrate and the III group nitride semiconductor crystal and due to thedifference in the linear expansion coefficients therebetween.

The III group nitride semiconductor substrate according to the presentinvention can be fabricated by the aforementioned fabrication method. Inother words, the first III group nitride semiconductor substrateaccording to the present invention is characterized by that a III groupnitride semiconductor crystal is formed via a metal film with meshstructure in which micro voids are provided. In addition, the second IIIgroup nitride semiconductor substrate according to the present inventionis characterized by that a III group nitride semiconductor crystal isformed via a metal nitride film with mesh structure in which micro voidsare provided.

As described above, the patent documents 5 to 8 disclose methods forgrowing a III group nitride semiconductor crystal via a titanium nitrideor the like. However, this titanium nitride is used for the purpose ofreducing the lattice mismatch between the intervening layer and the IIIgroup nitride semiconductor crystal. On the other hand, the fabricationmethod according to the present invention is characterized by that themicro voids are provided in the metal film or the metal nitride film andthe voids are provided with the nuclei generating site for the III groupnitride semiconductor crystal. In other words, the metal film or themetal nitride film and the III group nitride semiconductor crystal arenot continuously lattice matched at the interface therebetween.Therefore, the fabrication method according to the present inventiondoes not depend upon the lattice matching between the metal film or themetal nitride film and the III group nitride semiconductor crystal, thatis not likely to the methods described in the aforementioned patentdocuments, and the function and effect of the metal film or the metalnitride film are also different therefrom.

Further, as for a mask disclosed in the patent document 9, an effect ofproviding the nuclei generating site to a limited extent like thefabrication method according to the present invention cannot beexpected. In the fabrication method according to the present invention,the metal film or the metal nitride film having the micro voids can beself-assembled by a heat treatment of the metal film, which is a simpleand easy method without the need for the photolithography process, sothat insertion of the low-temperature buffer layer is not necessary, andthe nuclei generating site for the III group nitride semiconductorcrystal can be provided to a limited extent.

According to the present invention, the metal film or the metal nitridefilm in which the micro voids are formed is formed on the startingsubstrate, so that the crystal growth of the III group nitridesemiconductor is started from the voids. The micro voids formed in themetal film or the metal nitride film function as the nuclei generatingsite at the initial growth of the III group nitride semiconductorcrystal, and relax the strain due to the lattice mismatch between thestarting substrate and the III group nitride semiconductor crystal anddue to the difference in the linear expansion coefficients therebetween.Therefore, the III group nitride semiconductor substrate with lowdislocation density and less warping can be obtained without using thelow-temperature growth buffer layer.

The III group nitride semiconductor substrate according to the presentinvention can be broadly used as a substrate for a GaN-based device.Particularly, when used as the substrate for a laser diode or a highpower light emitting diode device, a high quality GaN-based crystal withlow dislocation density can be provided, so that it is possible tofabricate a device with high reliability and high performance.

In addition, according to the present invention, by providing a surfaceof the starting substrate (a forming surface of the metal film or themetal nitride film) as a nonpolar forming surface, it is possible toprovide a III group nitride semiconductor substrate, a substrate for aIII group nitride semiconductor device and fabrication methods thereof,each of which is provided with a high quality III group nitridesemiconductor crystal having a nonpolar surface at its surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing an example of IIIgroup nitride semiconductor substrate according to the presentinvention;

FIG. 2 is a process cross-sectional view showing an example of a methodfor fabricating the III group nitride semiconductor substrate accordingto the present invention;

FIG. 3 is a process cross-sectional view showing another example of themethod for fabricating the III group nitride semiconductor substrateaccording to the present invention;

FIG. 4 is a scanning electron microscope (SEM) photograph showing asurface of a Pt film with mesh structure in the EXAMPLE 1;

FIG. 5 is a scanning electron microscope (SEM) photograph showing asurface of a TiN film with mesh structure in the EXAMPLE 2;

FIG. 6 is a scanning electron microscope (SEM) photograph showing asurface of a TiN film with mesh structure in the EXAMPLE 3;

FIG. 7 is a schematic cross-sectional view showing a substrate for LEDwhich is an example of a substrate for a III group nitride semiconductordevice according to the present invention; and

FIG. 8 is a schematic cross-sectional view showing a LD devicefabricated by using a substrate for a LD which is an example of thesubstrate for a III group nitride semiconductor device according to thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an example of III group nitride semiconductor substrate ofthe present invention. A metal film or a metal nitride film 2′ with meshstructure in which micro voids are provided is formed on a sapphiresubstrate 1, and a III group nitride semiconductor crystal layer 3 isformed via the metal film or the metal nitride film 2′. The method offabricating a III group nitride semiconductor substrate according to thepresent invention may adopt various arrangements for respectiveprocesses or materials to be used for the III group nitridesemiconductor substrate. It is preferable that the metal film or themetal nitride film 2′ is formed on an entire surface of a startingsubstrate (sapphire substrate 1). According to this structure, the microvoids can be formed over the entire surface of the substrate, therebygrowing the III group nitride semiconductor crystal layer 3homogeneously as well as relaxing the strain more effectively.

It is preferable that the metal film or the metal nitride film satisfiesthe following conditions.

(i) A melting point or a decomposition starting temperature of the metalfilm or the metal nitride film is higher than a growth temperature ofthe III group nitride semiconductor formed thereon, and a form of thefilm can be kept at the growth temperature.

(ii) A vapor pressure at the growth temperature for the III groupnitride semiconductor layer is low enough, and sublimation does notoccur in the growth temperature.

(iii) In the growth temperature for the III group nitride semiconductorlayer, the metal film or the metal nitride film does not react with anitride semiconductor and source gas thereof, or the gas (ammonia gas orhydrogen gas) in a growth atmosphere, so that the crystal orientation isnot perturbed.

(iv) The metal film or the metal nitride film can transmit theorientation of the starting substrate, and can be oriented in the [0001]axis direction if it is a hexagonal system on the starting substrate andcan be oriented in the [111] axis direction if it is a cubic system.

The III group nitride semiconductor substrate having an excellentcrystal quality can be obtained by forming the metal film or the metalnitride film satisfying the above conditions.

The metal film or the metal nitride film may be a single film or acomposite film in which more than two kinds of layers are laminated. Forthe metal nitride film, a metal film comprising metallic element whichis easy to be nitrided is used, and the nitriding treatment and the heattreatment can conducted simultaneously by exposing it to the growth gasatmosphere for the III group nitride semiconductor layer. For themetallic element composing the metal film, titanium, tantalum, tungstenor the like are preferable. For this case, it is not absolutelynecessary to provide a process for nitriding, and a process forcontrolling a degree of the nitriding may be provided independently. Forthe method for forming the metal film, vacuum deposition, sputteringmethod, various chemical vapor depositions or the like can be used.

When titanium is selected as the material of the metal film, the heattreatment is preferably conducted at 700 to 1400° C. to nitride themetal film and form substantially homogeneous voids, and more preferablyat 800 to 1200° C. If it is lower than 700° C., the nitriding reactionwill not progress enough and the substantially homogeneous voids cannotbe formed. In addition, if the temperature exceeds 1400° C., the metalnitride film will not be flat. It is preferable to conduct the heattreatment at 700 to 1200° C., when a nitride semiconductor substrate ora nitride semiconductor epitaxial wafer is used as the startingsubstrate. If the temperature exceeds 1200° C., the heat decompositionof the single crystal gallium nitride layer progresses in excess, andthere is a risk that metal nitride film may be exfoliated.

For the method for growing the III group nitride semiconductor layer,various kinds of methods including MOCVD method (Metalorganic chemicalvapor deposition method), MBE method (molecular beam epitaxy method),HVPE method (Hydride vapor phase epitaxy method) or the like may beused. It is preferable to use the HVPE method for growing a thick filmof the III group nitride semiconductor so as to obtain an ingot of theIII group nitride semiconductor. It is because that the crystal growthspeed is high and the thick film can be easily obtained. However, thepresent invention is not limited to the HVPE method, and other methodsincluding the MOCVD methods or the like may be used, further, thecombination of a plurality of growth methods may be used (the III groupnitride semiconductor may be grown by using the MOCVD method halfway,and the III group nitride semiconductor may be grown to be thick byusing the HVPE method thereafter).

For the growth of the III group nitride semiconductor layer, inert gasor mixed gas of the inert gas and hydrogen or the like may be used as acarrier gas. For the inert gas, at least one kind of gas selected from agroup composed of N₂, He, Ne, Ar, Kr and Xe may be used. In the case offorming the III group nitride layer, the inert gas of nitrogen or thelike may be used as the carrier gas at the initial growth, and thecarrier gas may be changed to the hydrogen thereafter to grow a layerwith excellent crystallinity.

For the starting substrate, a substrate comprising various kinds ofmaterials may be used. As a preferred substrate, a different materialsubstrate such as sapphire, silicon, SiC, Langasite, Al, GaAs, γ-LiAlO₂or the like, and a substrate comprising a III group nitridesemiconductor such as GaN, AlN, AlGaN or the like may be proposed. Whenthe sapphire is used as the substrate material, C-plane, A-plane,R-plane or the like may be used. It does not matter if the startingsubstrate has the off-angle, however, it is preferable that theoff-angle is within 1°. By keeping the off-angle within 1°, theorientation of the metal film or the metal nitride film can be kept welland the III group nitride semiconductor layer can be grown well.

In addition, by providing the surface of the starting substrate (theforming surface of the metal film or the metal nitriding film) as thenonpolar forming surface, a high quality III group nitride semiconductorcrystal having the nonpolar surface at its surface can be obtained. Acrystal plane to be used for the nonpolar forming surface may be changedin accordance with the substrate material for the starting substrate.For example, when the starting substrate is sapphire, the nonpolarforming surface will be the r-plane, when the starting substrate isγ-LiAlO₂, the (100) plane will serve as the nonpolar forming surface,when the starting substrate is SiC, the a-plane or the m-plane willserve as the nonpolar forming surface, and when the starting substrateis a III group nitride semiconductor, the nonpolar surface (i.e. thea-plane or the m-plane) will serve as the nonpolar forming surface.

The III group nitride semiconductor layer may comprise various kinds ofsemiconductor layers. For example, a semiconductor layer such as GaN,AlGaN, InGaN, InAlGaN or the like may be used. In addition, theself-standing substrate of the III group nitride semiconductor can beobtained by exfoliating and removing the starting substrate afterforming the III group nitride semiconductor layer. In the presentspecification, the word “self-standing substrate” means a substratehaving the strength enough to hold its proper configuration as well asthe strength in which any inconvenience does not occur for handling. Fora method for removing the starting substrate, a method of mechanicallyexfoliating the starting substrate by applying a stress to a portionhaving a gap in a wafer after growth, a method of exfoliating thestarting substrate by removing a metallic element-containing film or aregion provided with a gap in the III group nitride semiconductor layerby etching, or the like may be used.

The method for fabricating a III group nitride semiconductor substrateaccording to the present invention may be modified to provide anyvariations other than the aforementioned configurations, within a rangewhich does not go beyond the object of the present invention. Forexample, it may be applied to the fabrication of a single crystalself-standing substrate of a ternary mixed crystal such as AlGaN, InGaN,or the like, and the fabrication of a p-type GaN substrate doped with Mgor the like. In the III group nitride semiconductor substrate accordingto the present invention, the micro voids provided in the metal film orthe metal nitride film may be formed by the photolithography or electronbeam lithography after the film formation as well as the aforementionedself-assembling technique which is simple and easy.

EXAMPLES

The present invention will be explained in more detail in the followingexamples, but the present invention is not limited thereto.

Example 1

In accordance with fabrication process shown in FIG. 2, a III groupnitride semiconductor substrate as below was fabricated. At first, ametal (Pt) film 2 having a thickness of 30 nm was vacuum-evaporateddirectly on a C-plane of a single crystal sapphire substrate 1 having adiameter of 2 inches as a starting substrate. This substrate was placedin an electric furnace, and the heat treatment was conducted at 1150° C.for 30 minutes in a flow of N₂ gas, as a result, the evaporated film ofPt was self-assembled by agglomeration mechanism to provide a metal (Pt)film 2′ with mesh structure comprising numerous micro voids. A surfaceof the metal (Pt) film 2′ with mesh structure was observed by theelectron microscope, and as shown in FIG. 4, and it was confirmed thatthe mesh structure in which elongated voids each having a width of 3 to5 μm and branching locally (an area of each opening is 1×10⁻⁹ m² orless) are substantially homogenously distributed over the entire surfaceof the substrate. A total area of the openings of the voids obtained byobserving the surface by the electron microscope was 60 to 70 area %,where the surface area of the starting substrate is defined as 100 area%.

The substrate on which the metal (Pt) film 2′ with mesh structure wasformed was placed in a HVPE furnace, and the substrate temperature wasincreased up to 1050° C., then a GaN layer 3 was grown directly on thesubstrate by using GaCl and NH₃ as source and SiH₂Cl₂ as dopant to havea thickness of 500 μm. The growth was conducted at a normal pressure anda mixed gas of 10 volume % of H₂ and 90 volume % of N₂ was used as thecarrier gas. In addition, a GaCl partial pressure and a NH3 partialpressure in a feed gas were 8×10⁻³ atm and 8×10⁻² atm, respectively. AGaN epitaxial growth substrate with a configuration shown in FIG. 1 wasobtained by the aforementioned process.

The X-ray diffraction analysis of the GaN epitaxial growth substratethus obtained was conducted, and it was confirmed that the grown GaNlayer 3 was a single crystal having a (0001) plane at its surface. Inaddition, half bandwidths of rocking curves having peaks at (0002) and(10-10) were measured, and good values of 220 seconds and 450 secondsrespectively were shown.

Example 2

In accordance with a fabrication process shown in FIG. 2, a III groupnitride semiconductor substrate as below was fabricated. At first, ametal (Ti) film 2 having a thickness of 20 nm was vacuum-evaporated byusing an EB evaporator directly on a C-plane of a single crystalsapphire substrate 1 having a diameter of 2 inches as a startingsubstrate. This substrate was placed in an electric furnace, and theheat treatment was conducted at 1050° C. for 30 minutes in a flow of amixed gas of H₂ gas and NH₃ gas, as a result, the evaporated film of Tiwas self-assembled by agglomeration mechanism into mesh structurecomprising numerous micro voids and nitrided by NH₃ into TiN, to providea metal nitride (TiN) film 2′ with mesh structure. A surface of themetal nitride (TiN) film 2′ with mesh structure was observed by theelectron microscope, and as shown in FIG. 5, and it was confirmed thatthe mesh structure in which elongated voids each having a width of 30 to50 nm and branching locally (an area of each opening is 1×10⁻⁹ m² orless) were substantially homogenously distributed over the entiresurface of the substrate. In addition, particular changes were notadmitted in the starting sapphire substrate. A total area of theopenings of the voids obtained by observing the surface by the electronmicroscope was 45 to 55 area %, where the surface area of the startingsubstrate is determined as 100 area %. The X-ray diffraction analysis ofthis substrate was conducted, a diffraction peak of TiN was observed,and it was confirmed that the metal nitride (TiN) film 2′ with meshstructure was a film oriented in a [111] direction.

The substrate on which the metal nitride (TiN) film 2′ with meshstructure was formed was placed in a MOVPE furnace, and the substratetemperature was elevated up to 1050° C., then an undoped GaN layer 3 wasgrown on the substrate by using trimethylgallium (TMG) and NH₃ as sourceto have a thickness of 2 μm.

A GaN epitaxial growth substrate with a configuration shown in FIG. 1was obtained by the aforementioned process. The surface of the GaNepitaxial growth substrate thus obtained has a mirror surface at itssurface, and it was confirmed by microscope observation that thesubstrate was a substantially flat film with good quality except thatsome terrace-shaped morphology was observed.

The X-ray diffraction analysis of the GaN epitaxial growth substratethus obtained was conducted, and it was confirmed that the grown GaNlayer 3 was a single crystal having a (0001) plane at its surface. Inaddition, half bandwidths of rocking curves having peaks at (0002) and(10-10) were measured, and they were 280 seconds and 450 secondsrespectively, which are the values equal to or more than the values ofthe GaN film obtained by using the conventional low-temperature growthbuffer layer. In particular, it was observed that in the half bandwidthof the rocking curve having a peak at (10-10) was smaller than that ofthe crystal grown by the conventional method. This is an advantage ofthe GaN which was grown via the film with mesh structure comprisingmicro voids.

A surface of the GaN epitaxial substrate thus obtained was dipped into aheated mixed liquid of phosphoric acid and sulfuric acid (250° C.) andthe dislocation density at an etch pit was observed, as a result, it wasconfirmed that the substrate has high crystallinity in which thedislocation density was 8×10⁷ pcs/cm², which is low.

Example 3

For the purpose of observing a state of the initial crystal growth whenconducting the crystal growth of GaN in the MOVPE furnace, the substrateon which the metal nitride (TiN) film 2′ with mesh structure was formedin the process same as the EXAMPLE 2 was fabricated, then the growth inthe MOVPE furnace was conducted for about 5 minutes then stopped andquenched, and the state of the surface of a sample taken out therefromwas observed by the electron microscope. As shown in FIG. 6, the GaN hasstarted to be selectively grown from the mesh of TiN, and it was notobserved that the state where the crystal nuclei of the GaN wasgenerated on the TiN.

Example 4

In accordance with a fabrication process shown in FIG. 3, a III groupnitride semiconductor substrate as below was fabricated. At first, ametal (Ti) film 2 having a thickness of 20 nm was vacuum-evaporated byusing the EB evaporator directly on a C-plane of a single crystalsapphire substrate 1 having a diameter of 2 inches as a startingsubstrate. This substrate was placed in an electric furnace, and theheat treatment was conducted at 1050° C. for 30 minutes in a flow ofmixed gas of H₂ gas and NH₃ gas, as a result, the evaporated film of Tiwas self-assembled by agglomeration mechanism into mesh structurecomprising micro voids and nitrided by NH₃ into TiN, to provide a metalnitride (TiN) film 2′ with mesh structure.

The substrate on which the metal nitride (TiN) film 2′ with meshstructure was formed was placed in a HVPE furnace, and the substratetemperature was elevated up to 1040° C. in an atmosphere of N₂, then aGaN layer 3 was grown to have a thickness of about 3 mm to form a IIIgroup nitride semiconductor crystal layer (HVPE-GaN layer) 3. NH₃ andGaCl were used as the growth source. A GaCl partial pressure and a NH₃partial pressure in the feed gas were 8×10⁻³ atm and 5.6×10⁻² atm,respectively, and a V/III ratio was 7. The growth was conducted at thenormal pressure, and N₂ gas mixed with 10 vol % of H₂ was used as acarrier gas. The GaN was grown for about 20 μm under this condition,then the carrier gas was changed to H₂, and the GaN was sequentiallygrown and a thick film of the GaN layer 3 was obtained.

The thick film epitaxial wafer of GaN thus obtained was once stuck bywax on a ceramic piece which is a base, and sliced in parallel with asurface of GaN by a wire saw, so that a GaN self-standing single crystalsubstrate 4 was cut out. Since the surface of GaN thus obtained was inthe condition where hillocks or the like which are the morphology of theas-grown are left or in the irregular condition where saw marks areleft, the surface was finished flatly by using diamond abrasive grains.As a result, the GaN self-standing single crystal substrate 4 having themirror surface at its surface was obtained.

A surface of the GaN self-standing single crystal substrate 4 thusobtained was dipped into a heated mixed liquid of phosphoric acid andsulfuric acid (250° C.) and the dislocation density at an etch pit wasobserved, as a result, it was confirmed that the substrate has a highcrystallinity in which the dislocation density was 1×10⁷ pcs/cm², whichis very low.

Example 5

The GaN epitaxial substrate obtained in the EXAMPLE 2 was placed in aHVPE furnace, and the substrate temperature was elevated up to 1040° C.in an atmosphere of N₂, then a GaN layer 3 was grown to have a thicknessof about 15 mm to form a III group nitride semiconductor crystal layer(HVPE-GaN layer) 3. NH₃ and GaCl were used as the growth source. A GaClpartial pressure and a NH₃ partial pressure in the feed gas were 8×10⁻³atm and 5.6×10⁻² atm, respectively, and a V/III ratio was 7. The growthwas conducted at the normal pressure, and a N₂ gas mixed with 10 vol %of H₂ was used as a carrier gas.

The thick film epitaxial wafer of GaN thus obtained was once stuck bywax on a ceramic piece which is a base, and sliced in parallel with asurface of GaN by a wire saw, and a GaN self-standing single crystalsubstrate was cut out. Since front and back surfaces of GaN thusobtained are in the condition where hillocks or the like which are themorphology of the as-grown are left or in the irregular condition wheresaw marks are left, the surface was finished flatly by using diamondabrasive grains. As a result, the GaN self-standing single crystalsubstrate 4 having the mirror surface at is surface was obtained. TheGaN self-standing substrate thus obtained was colorless and transparent.

Example 6

A III group nitride semiconductor substrate was fabricated similarly tothe EXAMPLE 1, except that a sapphire substrate in which an A-plane isthe growth surface was used as a starting-substrate instead of thesapphire substrate in which a C-plane is the growth surface. Incomparison with the EXAMPLE 1 in which the C-plane sapphire was used,the III group nitride semiconductor substrate having a larger diametercould be easily obtained.

Example 7

A III group nitride semiconductor substrate was fabricated similarly tothe EXAMPLE 1, except that aluminum gallium nitride (AlGaN) was growninstead of growing GaN. Growth conditions of the AlGaN are as follows. Asubstrate temperature was 1060° C., TMG, trimethylaluminum (TMA) and NH₃were used as growth source, a mixed gas of 40 vol % of H₂ and 60 vol %of N₂ was used as a carrier gas, and TMG partial pressure, TMA partialpressure and NH₃ partial pressure were 6.5×10⁻ ⁷ atm, 1.0×10⁻⁷ atm and0.1 atm, respectively. It was confirmed by the X-ray diffractionanalysis that the substrate thus obtained was a single crystal substratesimilarly to the substrate in the EXAMPLE 1.

Example 8

A III group nitride semiconductor substrate was fabricated similarly tothe EXAMPLE 1, except that indium gallium nitride (InGaN) was growninstead of growing GaN. Growth conditions of the InGaN are as follows. Asubstrate temperature was 750° C., TMG, trimethylindium (TMI) and NH₃were used as growth source, a mixed gas of 2 vol % of H₂ and 98 vol % ofN₂ was used as carrier gas, and TMG partial pressure, TMA partialpressure and NH₃ partial pressure were 6.5×10⁻⁷ atm, 5×10⁻⁸ atm and 0.1atm, respectively. It was confirmed by the X-ray diffraction analysisthat the substrate thus obtained was a single crystal substratesimilarly to the substrate in the EXAMPLE 1.

Example 9

A III group nitride semiconductor substrate was fabricated similarly tothe EXAMPLE 1, except that a nickel film, a tungsten film and amolybdenum film were respectively formed as metal films instead of thePt film. The nickel film, tungsten film and molybdenum film after theheat treatment were observed by the electron microscope, and the meshstructure same as that in the Pt film in the EXAMPLE 1 was admitted. TheX-ray diffraction analysis was conducted for the substrate in which theGaN layer was grown on the substrate where the metal film 2′ with meshstructure was formed, and it was confirmed that the GaN layer was asingle crystal in case where any metal film was used.

Example 10

A III group nitride semiconductor substrate was fabricated similarly tothe EXAMPLE 2, except that the porous metal nitride (TiN) film wasformed by providing a TiN film by sputtering method and forming microvoids on an entire surface of the TiN film by photolithography. Thesubstrate thus obtained was a single crystal substrate having a highcrystallinity same as the substrate in the EXAMPLE 2.

Example 11

A III group nitride semiconductor substrate was fabricated similarly tothe EXAMPLE 1, except that a r-plane of a sapphire substrate was used asa starting-substrate instead of a C-plane of the sapphire substrate.

The X-ray diffraction analysis of the GaN epitaxial growth substratethus obtained was conducted and it was confirmed that the grown GaNlayer 3 was the single crystal having a-plane ((11-20) plane) which wasa nonpolar surface at its surface. In addition, half bandwidth of arocking curve measured by the X-ray diffraction was 350 seconds, whichwas a good value.

In addition, III group nitride semiconductor substrates were madesimilarly by using (100) plane of γ-LiAlO₂ substrate, a-plane of a SiCsubstrate, a-plane of a GaN substrate respectively instead of using thesapphire substrate as the starting substrate, and it was confirmed thatthey have nonpolar surfaces at their surfaces similarly.

Example 12

A III group nitride semiconductor substrate was fabricated similarly tothe EXAMPLE 2, except that a r-plane of the sapphire substrate was usedas a starting-substrate instead of a C-plane of the sapphire substrate.

The X-ray diffraction analysis of the GaN epitaxial growth substratethus obtained was conducted and it was confirmed that the grown GaNlayer 3 was the single crystal having a-plane ((11-20) plane) which wasa nonpolar surface at its surface. In addition, half bandwidth of arocking curve measured by the X-ray diffraction was 380 seconds, whichwas a good value. In addition, III group nitride semiconductorsubstrates were made similarly by using (100) plane of γ-LiAlO₂substrate, a-plane of a SiC substrate, a-plane of a GaN substraterespectively instead of using the sapphire substrate as the startingsubstrate, and it was confirmed that they have nonpolar surfaces attheir surfaces similarly.

Example 13

FIG. 7 shows a substrate for a LED in one embodiment of the substratefor a III group nitride semiconductor device according to the presentinvention. A substrate 18 for a LED comprises a substrate 13 forepitaxial growth in which a metal (Pt) film 12 with mesh structure isformed on a sapphire substrate 11. This substrate 13 for epitaxialgrowth was fabricated similarly to the EXAMPLE 1, except that a r-planeof a sapphire substrate was used as the starting substrate instead ofusing a C-plane of the sapphire substrate.

On this substrate 13 for epitaxial growth, a n-type GaN layer 14 havinga thickness of 4 μm, an In_(0.2)Ga_(0.8)N/GaN-3-MQW active layer (a welllayer of 3 nm, a barrier layer of 10 nm) 15, a p-type Al_(0.1)Ga_(0.9)Nlayer 16 having a thickness of 40 nm, and a p-type GaN layer 17 having athickness of 500 nm are sequentially laminated by using the MOVPE methodto provide a plurality of III group nitride semiconductor epitaxiallayers having a LED structure.

The X-ray diffraction analysis was conducted for a surface of the p-typeGaN layer 17, and it was confirmed that it has a-plane ((11-20) face)which was a nonpolar surface at its surface.

Crystallinity of the epitaxial layer was good, and an emission power ofthe LED was also good.

According to this substrate 18 for a LED, a nanomask structure of themetal (Pt) film 12 which is an interface between the n-type GaN layer 14and the sapphire substrate 11 shows an effect like a photonic crystal,and light generated at the active layer 15 passes through withoutreflecting at the interface. Accordingly, much light enters into thesapphire substrate 11 in which a difference in refractive indices withthe air is small, and there is an advantage in that efficiency fortaking out the light is improved.

Example 14

In the substrate 18 for a LED shown in FIG. 7 explained in the EXAMPLE13, a metal nitride (TiN) film 12 with mesh structure was formed insteadof the metal (Pt) film 12 with mesh structure to fabricate the substrate18 for a LED.

The substrate 13 for epitaxial growth in which the (TiN) film 12 withmesh structure was formed on the sapphire substrate 11 was fabricated bythe same process as that in the EXAMPLE 2, except that the r-plane ofthe sapphire substrate was used as the starting substrate instead of theC-plane of the sapphire substrate. The formation of the epitaxial layerfor a LED on the substrate 13 for epitaxial growth was conductedsimilarly to the EXAMPLE 13.

In this substrate 18 for a LED, it was also found that there arecharacteristics and effects similar to those in the EXAMPLE 13.

Example 15

FIG. 8 shows a LD device fabricated by using a substrate for a LD in oneembodiment of the substrate for a III group nitride semiconductor deviceaccording to the present invention.

A LD device 32 comprises a substrate 23 for epitaxial growth in which ametal (Pt) film 22 with mesh structure is formed on a sapphire substrate21. This substrate 23 for epitaxial growth was made similarly to theEXAMPLE 1, except that the r-plane of a sapphire substrate was used asthe starting substrate instead of using the C-plane of the sapphiresubstrate.

On this substrate 23 for epitaxial growth, a n-type GaN base layer 24having a thickness of 4 μm and a carrier concentration of 1×10¹⁸, an-type Al_(0.07)Ga_(0.93)N cladding layer 25 having a thickness of 1 μmand a carrier concentration of 5×10¹⁷, an undopedIn_(0.1)Ga_(0.9)N/In_(0.02)Ga_(0.98)N-3-MQW active layer (a well layerof 4 nm, a barrier layer of 4 nm) 26, and a p-type Al_(0.07)Ga_(0.93)Ncladding layer 27 having a thickness of 0.5 μm and a carrierconcentration of 5×10¹⁷ are sequentially laminated by using the MOVPEmethod to form a plurality of III group nitride semiconductor epitaxiallayers having a LD structure. The product fabricated at this stage isthe substrate for a LD. The product fabricated by conducting an ordinaryLD device fabricating process such as etching or electrode formation isa LD device 32 shown in FIG. 8. In the drawing, 28 indicates a narrowstripe-shaped waveguide for a semiconductor laser, 29 indicates a Ni/AuOhmic contact, 30 indicates a Ti/Al Ohmic contact, and 31 indicates aSiO₂ film.

The X-ray diffraction analysis of a surface of p-typeAl_(0.07)Ga_(0.93)N cladding layer 27 thus obtained was conducted and itwas confirmed that it has an a-plane ((11-20) plane) which was anonpolar surface at its surface.

Crystallinity of the epitaxial layer was good, and there was no Starkeffect, since the LD structure comprises the epitaxial layer having thenonpolar surface, and the LD device having the high emission efficiencycould be obtained.

Example 16

In the LD device 32 shown in FIG. 8 explained in the EXAMPLE 15, themetal nitride (TiN) film 22 with mesh structure was formed instead ofthe metal (Pt) film 22 with mesh structure to fabricate the LD device32.

The substrate 23 for epitaxial growth, in which the (TiN) film 22 withmesh structure was formed on the sapphire substrate 21, was fabricatedby the same process as that in the EXAMPLE 2, except that the r-plane ofthe sapphire substrate was used as the starting substrate instead of theC-plane of the sapphire substrate. The formation of the epitaxial layerfor a LED on the substrate 23 for epitaxial growth was conductedsimilarly to the EXAMPLE 15.

In this LED device 32, it was also found that there are characteristicsand effects similar to those in the EXAMPLE 15.

Although the invention has been described with respect to specificembodiment for complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodification and alternative constructions that may be occurred to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

1. A method for fabricating a III group nitride semiconductor substrate,comprising the steps of: forming a metal film on a starting substrate;conducting a heat treatment for the starting substrate on which themetal film is formed to form innumerable micro voids which extend from asurface of the metal film to a surface of the starting substrate in themetal film; and growing a III group nitride semiconductor crystal viathe metal film on the heat treated starting substrate, wherein thesurface of the starting substrate (a forming surface of the metal film)is a nonpolar forming surface for providing a growth surface of the IIIgroup nitride semiconductor crystal as a nonpolar surface, and thegrowth surface of the III group nitride semiconductor crystal is thenonpolar surface.
 2. The method for fabricating a III group nitridesemiconductor substrate, according to claim 1, wherein: a devicestructure comprising a plurality of III group nitride semiconductorepitaxial layers is formed in the process for growing the III groupnitride semiconductor crystal.
 3. A method for fabricating a III groupnitride semiconductor substrate, comprising the steps of: forming ametal film on a starting substrate; conducting a heat treatment for thestarting substrate on which the metal film is formed in an atmosphere ofgas containing a nitrogen element at a temperature of 800° C. or more toform a metal nitride film by nitriding the metal film and to forminnumerable micro voids which extend from a surface of the metal nitridefilm to a surface of the starting substrate in the metal nitride film;and growing a III group nitride semiconductor crystal via the metalnitride film on the heat treated starting substrate, wherein the surfaceof the starting substrate (a forming surface of the metal nitride film)is a nonpolar forming surface for providing a growth surface of the IIIgroup nitride semiconductor crystal as a nonpolar surface, and thegrowth surface of the III group nitride semiconductor crystal is thenonpolar surface.
 4. The method for fabricating a III group nitridesemiconductor substrate, according to claim 3, wherein: a devicestructure comprising a plurality of III group nitride semiconductorepitaxial layers is formed in the process for growing the III groupnitride semiconductor crystal.